The present invention relates to a line pressure control system for an automatic transmission for a vehicle.
Japanese Patent Application Laid-Open No. 57-101154 discloses a control system in which a line pressure control valve is provided for controlling line pressure of an automatic transmission by an output signal from a computer such that the line pressure is controlled in proportion to throttle valve opening degree and vehicle speed. Generally, the electronic control system is arranged such that if supply voltage from a battery drops because of increase of engine load and inactivation of the battery at cold engine, the microcomputer is reset. In the reset state, the line pressure is not controlled by the microcomputer. In order to ensure the starting of the engine and driving of the vehicle, an electrical circuit is provided to control the line pressure to a maximum value. However, at cranking of the engine, if the line pressure is high, pumping loss of an oil pump becomes large. The pumping loss affects a starting of the engine. Therefore, it is preferable that the line pressure is low at cranking in order to prevent the pumping loss.